1. Field of the Invention
The present invention relates to a method for fabricating a thin-film structure body, for example, suitable for fabrication of a semiconductor sensor in which a beam structure composed of a thin film is formed on a semiconductor substrate to detect a physical quantity such as acceleration, yaw rate, vibration, or the like by displacement of the beam structure.
2. Description of the Related Art
Recently, the demand has grown for a more compact and lower cost semiconductor acceleration sensor. To this end, a differential-capacitance type semiconductor acceleration sensor that employs polycrystalline silicon as an electrode is disclosed in PCT WO 92/03740. A sensor of this type is described utilizing FIG. 33 and FIG. 34. FIG. 33 indicates a plan view of the sensor, and FIG. 34 is a C--C sectional view of FIG. 33.
A movable member 116 of beam structure is disposed above a silicon substrate 115 with a predetermined gap interposed therebetween. The movable member 116 composed of polycrystalline silicon thin film comprises beam sections 121 and 122, a weight section 123, and movable electrode sections 124. The movable member 116 is fixed on an upper surface of the silicon substrate 115 by anchor sections 117, 118, 119, and 120. That is to say, the beam sections 121 and 122 extend from the anchor sections 117, 118, 119, and 120 of the movable member 116, and the weight section 123 is supported by these beam sections 121 and 122. The movable electrode sections 124 are formed on this weight section 123. Two fixed electrodes 125 are disposed on the silicon substrate 115 and oppose the movable electrode section 124. Accordingly, the structure is configured so when acceleration is applied in a direction parallel to the surface of the silicon substrate 115 (indicated by X in FIG. 33), the electrostatic capacitance between the movable electrode portion 124 and the fixed electrodes 125 increases on one side and decreases on the other.
In fabrication of this sensor, as shown in FIG. 35, a sacrificial layer 126 of silicon-oxide film or the like is formed on the silicon substrate 115, and along with holes 127 are formed in the sacrificial layer 126 at places which become anchor sections. Thereafter, as shown in FIG. 36, a polycrystalline silicon film 128 which becomes the movable member 116 is deposited on the sacrificial layer 126 and configured in the specified pattern. Furthermore, as shown in FIG. 37, the sacrificial layer 126 below the movable member 116 is etched away with an etchant, and the movable member 116 is disposed above the silicon substrate 115 with a predetermined gap interposed therebetween.
However, as shown in FIG. 38, during film formation, internal stress .sigma. is exerted from the interface of the sacrificial layer 126 to the movable member 116 composed of polycrystalline silicon thin film, and internal stress .sigma. gradually changes and increases in the direction of film thickness. As a result, internal stress distribution exists in the direction of film thickness of the movable member 116, and the resulting movable member is warped. That is to say, as shown in FIG. 33, the movable electrode section 124 assumes a cantilever structure taking the weight section 123 as a fixed end, and the movable electrode section 124 is warped due to internal stress distribution existing in the direction of film thickness. As a result, the movable electrode section 124 and the fixed electrode 125 could not be disposed opposite each other with good precision. Additionally, deflection due to internal stress distribution is generated in the weight section 123 as well. As a result, the movable electrode section 124 which protrudes from this weight section 123 is also displaced and the movable electrode section 124 and the fixed electrode 125 could not be disposed opposite each other with good precision.
As a general means to reduce internal stress of a film structure body such as this, a long-term, high-temperature heat treatment on the film structure body (for example 24 hours at 1,150.degree. C.) is performed. However, this method could not be combined with an IC process because of the resulting damage on transistors and the like peripheral structure circuitry provided in the periphery of the movable member 116 on the silicon substrate 115. Furthermore, application in a semiconductor substrate acceleration sensor integrated with peripheral circuitry was not practical.